A conventional papermaking machine is divided into three primary sections, namely a forming section, a press section, and a drying section. In the forming section, a porous forming fabric is employed which is designed to receive an aqueous dispersion of fibers where a major portion of the water is removed through the forming fabric, leaving the fibers in sheet form, though at that stage without any real coherence between the fibers. In other words, inital formation of the paper sheet occurs in the forming section. Following initial formation of the sheet, the sheet is transferred to the press section where it is delivered onto a press felt, and while in contact with the felt moves through one or more press rolls or pressure nips for removal of most of the remaining water from the sheet. From the press section, the sheet then moves to the dryer section where the now low moisture paper sheet or web encounters a series of heated dryer cylinders with dryer fabrics therearound to further remove moisture from the web and dry the paper.
In each of the forming section, press section and drying section, a fabric or felt of some sort is employed to both support the sheet being formed and to facilitate removal of water therefrom. The present invention is directed to felts for use in the press section of the papermaking machine.
In the press section of the papermaking machine, the fabric or felt that is employed encircles a plurality of guide rolls in an endless loop with one or more pressure nips, vacuum boxes, spray units or the like located around the loop, to facilitate further formation of the paper web by the removal of water therefrom and subsequent removal of water from the felt. In the press section of the papermaking machine, the fabric which is generally referred to as a press felt or wet felt originally was an all wool felt. In fairly recent history, however, due to cost, and technology advances, wool felts have been replaced with composite felts where base fabrics were produced and fiber batts were secured to one or both sides of the base fabric to achieve the intended technical purposes of the press section of the papermaking machine. Furthermore, increased machine speeds, the demands for higher quality paper, the demands for specialty papers and the like place more stringent requirements on the press felts. Hence, if proper attention is not given to the physical characteristics of such felts, poor quality paper can be produced, felt life can end prematurely, production expenses can escalate and the like, all of which can lead to disastrous results. Particularly, in the press section of the machine, as will be explained in more detail hereinafter, the press or wet felts are continuously compressed and relieved from load at high frequencies thus necessitating proper elongation, stretch, compression, and release properties as well as overall stability and runability.
In attempting to arrive at improved press felts, various and sundry techniques have been employed, in the selection of the particular yarns or yarn groups that are utilized in the weaving of the base fabric, the weave designs per se, the batt structures, techniques for securing the batts to the base fabric, and the like. Many different weave patterns have been employed in the production of both single layer and multilayer fabrics utilizing natural and synthetic yarns and blends. Synthetic yarns such as polyesters, polyamides, polypropylenes and the like, have been employed in monofilament, multifilament, spun yarn and piled monofilament form for both warp and shute ends. Moreover, while historically non-round synthetic monofilaments have been used in forming fabrics and dryer fabrics for use on papermaking machines, insofar as the press felts are concerned, non-round synthetic monofilaments have been only sparingly utilized, and then only in the shute or cross-machine direction. There is no knowledge of any use of non-round monofilaments in the warp or machine direction for press felts. Furthermore, the use of non-round synthetic monofilaments as a warp end in a press felt is not obvious due, in part, to the fact that needling of fiber batts to base press fabrics would be expected to excessively break needles during the needling process and/or split the generally flat monofilaments, both of which could be highly detrimental to the papermaking process.
One problem existing in certain papermaking today is to avoid press felt damage due to the action of chlorine on the components of the felt. In some operations, the presence of chlorine in the stock water is greater than others. Chlorine is added to the fiber dispersion or furnish to bleach the paper to be formed, such as in the production of bleached pulp where the influence of chlorine is perhaps most significant.
Faced with the problem, felt producers have historically relied upon nylon and polyethylene terephthalate fibers and monofilaments, knowing in advance that each type has its advantages and disadvantages. For example, while nylon dissolves in the presence of chlorine, its other major characteristics such as abrasion resistance, elasticity, and the like are generally superior to polyethylene terephthalate. Conversely, the polyethylene terephthalate fibers and filaments have good chlorine resistance. Normally, the industry has elected nylon, expecting a short useful life of the felt. In fact, in some machines the chlorine problem is so bad that the nylon press felts must be replaced about every seven days.
The press felts according to the present invention improve, if not overcome the above problems by providing a felt that affords the good properties of both the nylon and the polyethylene terephthalate. Greatly improved press felts are the result.
There is thus no known prior art that is believed to anticipate or suggest the present invention.